Revenant Gamemode (#10851)

Added a new wave defense gamemode, Revenants, wherein the station crew have to defend themselves from an endless wave of humanoid bluespace creatures.

Ported the icon from Baystation, it's their blueforged, but I tweaked the mob's alpha values a tad. Baystation12/Baystation12#21669

code/modules/ghostroles/spawner/base.dm

@@ -2,7 +2,10 @@
 	var/short_name = null
 	var/name = null
 	var/desc = null
+
+	var/observers_only = FALSE
 	var/show_on_job_select = TRUE // Determines if the ghost spawner role is considered unique or not.
+
 	var/welcome_message = null
 	var/list/tags = list() //Tags associated with that spawner
 
@@ -71,6 +74,9 @@
 		if(!is_alien_whitelisted(user, req_species_whitelist))
 			return "Missing Species Whitelist"
 
+	if(observers_only && !isobserver(user))
+		return "Observers Only"
+
 	return FALSE
 
 //Return a error message if the user CANT spawn. Otherwise FALSE